1. Field of the Invention
The present invention relates to a process for continuously producing a phase retarder film or sheet used in liquid crystal display devices, etc.
2. Description of the Related Art
Phase retarder films or sheets each comprising a uniaxially oriented thermoplastic resin film or sheet have been used as optical compensators to enhance display qualities of liquid crystal display devices.
A STN type liquid crystal display device using such phase retarder film(s) or sheet(s) as optical compensator(s) is advantageous in that it is lighter in weight, thinner and less expensive, but disadvantageous in that it has a narrow viewing angle characteristic and exhibits a low contrast ratio as compared with a STN double cell type liquid crystal display device using a liquid crystal cell as optical compensator.
These disadvantages have been considerably eliminated by, for example, laminating two plies of phase retarder films or sheets, but the viewing angle characteristic has not reached a satisfactory level.
The viewing angle characteristic of a liquid crystal display device using phase retarders as optical compensators greatly depends not only upon the angular dependence of the birefringence of the liquid crystal cell used therein but also upon the angular dependence of the retardation of the phase retarder. In conventional phase retarders, it is known that lower angular dependence of retardation gives a superior result.
The angular dependence of retardation of a phase retarder film or sheet is expressed by a retardation ratio R.sub.40 /R.sub.0. In this case, the retardation R.sub.40 is a value measured with the phase retarder film or sheet being tilted by 40.degree. from the horizontal condition by rotating around an axis which corresponds to the slow axis if the phase retarder consists of a thermoplastic resin having a positive intrinsic birefringence, or to the fast axis if the phase retarder consists of a thermoplastic resin having a negative intrinsic birefringence; the retardation R.sub.0 is a value measured with the phase retarder film or sheet not being tilted (i.e. arranged horizontally); and the measurement is made using a polarizing microscope equipped with a Senarmont compensator. As the retardation ratio of a phase retarder is closer to 1, its angular dependence of retardation is lower.
In order to make low the angular dependence of retardation of a phase retarder film or sheet, there were proposed, for example, the following methods.
A method of shrinking a film in a direction perpendicular to the stretching axis at the time of the uniaxial stretching [Japanese Patent Kokai (Laid-open) No. 2-191904].
A method of stretching a film whose molecules are orientated in a direction normal to the film surface [Japanese Patent Kokai (Laid-open) No. 2-160204].
A method of stretching a film produced from a molten polymer or a polymer solution under an electric field applied [Japanese Patent Kokai (Laid-open) No. 2-285303]. None of these methods, however, is sufficient in improvement of the angular dependence and application to mass production.
The present inventors have made an extensive study in order to solve the above problems. As a result, the present inventors have found that a phase retarder film or sheet having a low angular dependence of retardation can be continuously produced by the continuous thermal relaxation of a stretched thermoplastic resin film or sheet by continuously supplying the film or sheet to a heating zone in parallel with the stretching direction of the film or sheet and heating the film or sheet to a temperature not lower than the glass transition temperature of the resin while applying pressure to the surface of the film or sheet passing through the heating zone. The present inventors thus completed the present invention.